Lawyers have widened a class action launched over soy milk sold in Australia with high levels of iodine to include the Japanese manufacturer and exporter.

Bonsoy was recalled worldwide late in 2009 after it was discovered that one glass contained seven times the safe dose of iodine.

Maurice Blackburn Lawyers launched a class action in 2010 on behalf of hundreds of Australians who they say suffered severe thyroid problems and other chronic illnesses as a result of drinking the milk.

The class action began against Australian brand Spiral Foods but has now widened to include the two Japanese companies that manufactured and exported Bonsoy - Marusan-ai and Muso.

The amended statement of claim alleges the three companies ignored a 2006 test that revealed Bonsoy contained high levels of iodine.

Lawyers say the high levels stem from the reformulation of Bonsoy in 2003 to include the iodine-rich kombu, a type of kelp, as an alternative to using salt.

More than 600 people have joined the class action.

Maurice Blackburn senior associate Irina Lubomirska says the reformulated Bonsoy was on the market for six years before being recalled.

"We say that these three companies had test results in mid-2006 which showed Bonsoy contained extremely high levels of iodine, but they did nothing," she said in a statement.

"On at least three occasions they were contacted by customers expressing concerns about iodine content of Bonsoy and they did not act to ensure the product was safe.

"They have breached consumer protection laws in both Australia and Japan."

Ms Lubomirska says a quick internet search would have informed the companies the levels of iodine were not safe.

"It was not until Christmas 2009 that Bonsoy was recalled after health authorities discovered that one glass of the product contained seven times the upper safe dose of iodine for adults," she said.

"By that time the defective product had been on the market for six years and hundreds of Bonsoy consumers suffered thyroid illness as a result."

The case will be heard in court in March. No date has been set for a trial.